Confusion Approach to Porphyrinoid Chemistry - ACS Publications

Acc. Chem. Res. 2005, 38, 10-20

Confusion Approach to Porphyrinoid Chemistry

Chart 1

ALAGAR SRINIVASAN† AND HIROYUKI FURUTA*,†,‡ Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, Fukuoka 812-8581, Japan, PRESTO, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan Received June 9, 2004 ABSTRACT N-confused porphyrin (NCP) is a porphyrin isomer that is different largely from the parent porphyrin, particularly in the physical, chemical, structural, and coordination properties. Introduction of the confused pyrrole into the normal and expanded porphyrins leads to generation of the confused porphyrinoids having rich structural diversity. In this Account, we introduce a series of N-confused porphyrinoids recently synthesized and highlight their properties such as fusion, peripheral N coordination, supramolecular assemblies, anion binding, and singlet-oxygen sensitization.

Introduction Porphyrin, a representative of conjugated tetrapyrrolic macrocycles, has been a potent candidate for diverse applications in the areas of biology, medicine, material science and catalysis.1 Recently, syntheses of new porphyrin isomers continue to attract the attention of chemists because of their spectral, structural, chemical, and physical properties, in comparison with the parent porphyrin system. So far, two kinds of porphyrin isomers have been discovered. Both the isomers have a formula of C20H14N4 in their macrocyclic core with an 18 π-electronconjugated pathway. In the first category, all of the pyrrole nitrogens (N4) are inside the core that differs from the arrangement of the pyrrole-linking carbon atoms from that of normal porphyrin. The first isomer of such kind, porphycene, was synthesized by Vogel et al. in 1986.2 Since then, corrphycene,3 hemiporphycene,4 and isoporphycene5 have been reported. In the second category, one of the pyrrole rings is confused or inverted to form an N3C core, which is considered a “true porphyrin isomer,” because it contains four meso-carbon atoms as the same as the normal one. The first isomer of such kind, N-confused porphyrin (NCP), was independently reported in 1994 by us and Latos-Graz˘ yn ´ski’s group.6 Recently, second genera-

Alagar Srinivasan was born in India. In 2000, he received his Ph.D. from the Indian Institute of Technology, Kanpur, under the supervision of Prof. T. K. Chandrashekar. Presently, he is a postdoctoral fellow with Prof. H. Furuta, Kyushu University, Japan. Hiroyuki Furuta received his Ph.D. from Kyoto University (1986). After working as a postdoctoral fellow at the Mitsubishi-Kasei Institute of Life Science at Machida and the University of Texas at Austin, he became Associate Professor at Oita University (1992) and then at Kyoto University (1997). He moved to Fukuoka as a Professor of Chemistry at Kyushu University in 2002. His research interests cover synthesis of novel porphyrinoids for functional dyes, metal coordination chemistry for supramolecular architectures, and chemical evolution. 10

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tion of the confused isomers, doubly N-confused porphyrins (N2CP), was also synthesized.7,8 Apart from the isomers, various carbaporphyrinoids such as azuliporphyrin,9a carbaporphyrins,9b and dicarbaporphyrins9c were synthesized by Lash et al. (Chart 1). Because of the similar inner framework to NCPs, the coordination chemistry of these ligands is closely related but in no way parallel to the NCPs.9 Furthermore, heteroatom-containing porphyrins, in which one or more pyrrolic nitrogen atoms are replaced by O, S, Se, or Te, were reported first by Ulman and co´ ski et al.10b workers,10a and later by Latos-Graz˘ yn Research on the expanded analogues has also received considerable attention. The first expanded porphyrin, sapphyrin, wherein five pyrrole rings are connected through four meso-carbon atoms, was reported by Woodward in 1966.11 In series, the first carbasapphyrin was introduced by Lash et al.,12 and the core-modified Nconfused sapphyrin was synthesized by us along with Chandrashekar et al.13 The next higher homologues, pentaphyrin and hexaphyrin, were reported by Gossauer in 1983 (Chart 2).14 Since then, a series of expanded porphyrins have been reported.15 These macrocycles are widely used as receptors for anions,15 sensitizers for photodynamic therapy (PDT),16 and magnetic resonance imaging (MRI) contrasting agents.17 In this Account, we emphasized our recent results, especially focusing on the confused analogues. At first, we described the synthesis of meso-aryl N-confused porphyrin (NCP, 1) and one of its characteristic properties, NH tautomerism. Second, we focused on the second generation of NCP, such as cis- and trans-N2CP and their metal complexes. Third, we discussed the transformation of the confused unit into a fused one in NCP and N-confused pentaphyrin. Next, we described the doubly N-confused derivatives of pentaphyrin and hexaphyrin, particularly, their oxo derivatives and metal complexes. Finally, we * To whom correspondence [email protected]. † Kyushu University. ‡ PRESTO, JST. 10.1021/ar0302686 CCC: $30.25

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 2005 American Chemical Society Published on Web 12/03/2004

Confusion Approach to Porphyrinoid Chemistry Srinivasan and Furuta

Chart 2

Scheme 1

Scheme 2

adopted a [2+2] acid-catalyzed condensation reaction (Scheme 1b).22 The 1H NMR signals of inner CH and NH protons in 1a were resonated at -4.99 and -2.41 ppm in CDCl3, respectively, which indicates the formation of inner 3H (1-3H) tautomer.6a While, in deuterated dimethylformamide (DMF-d7), new tautomeric species [inner 2H tautomer (1-2H)] was formed and the corresponding signals were resonated at 0.76 and 2.27 ppm, respectively, along with the outer NH signal at 13.54 ppm (Scheme 2).23 In less polar solvents such as CHCl3, the electronic absorption spectrum displayed a strong Soret-like band at 439 nm and the last Q band at 725 nm. In polar solvents such as DMF, the color of NCTPP changed from red to green and the Soret band was bathochromic-shifted, while the Q bands were hypsochromic-shifted as compared with the CHCl3 solution. Both the tautomers were further confirmed by X-ray single-crystal analyses.23

Cis- and Trans-Doubly N-Confused Porphyrin (N2CP) focused on the supramolecular chemistry of NCP, mainly the role of unique confused pyrrole in the coordination chemistry, its anion-binding ability, and the attempt of using as sensitizers for the singlet-oxygen generation. For nomenclature, we used the same terms (normal, confused, inverted, and fused) here as we mentioned in our earlier article.18

N-Confused Porphyrins (NCP) The first NCP (1) was synthesized through the Rothemundtype reaction, namely, the acid-catalyzed condensation of pyrrole and benzaldehyde, with concurrent formation of normal porphyrin. The reported yields were low [Nconfused tetraphenylporphyrin (NCTPP, 1a), 5-7%, and N-confused tetratolylporphyrin (NCTTP, 1b), 4%] (Scheme 1a).6 In 1999, Lindsey and co-workers reported an improved synthesis of 1a by using methanesulfonic acid as a catalyst, and the yield was increased to 39%.19 Perusal of the literature also revealed the meso-free type of NCP synthesis, which was obtained through the [2+2] and [3+1] acid-catalyzed condensation of the respective precursors followed by oxidation.20,21 For the synthesis of pentafluorophenyl-substituted NCP (1c), Lindsey’s modified procedure was not efficiently helpful; thus, we

One of the special features of NCP chemistry that differs from normal porphyrins is its multivalent nature as a metal ligand. If such effects were observed for other NCPs, especially, for the porphyrin-like macrocycles with more than one confused pyrrole ring, it could provide a potent strategy for stabilizing higher oxidation states of metals. In this section, we presented the cis- and trans-doubly N-confused porphyrins (cis-N2CP7 and trans-N2CP8) and their metal complexes. Cis-N2CP (2) was synthesized via the acid-catalyzed condensation of pentafluorobenzaldehyde with N-confused dipyrromethane in CHCl3 containing a trace of EtOH. After oxidation with 2,3-dichloro-5,6-dicyano-1,4benzoquinone (DDQ), a greenish product 2 was obtained in 2% yield along with the major product corrorin (3), a corrole isomer, in 13% yield (Scheme 3).7a The β-CH protons of two confused pyrrole rings in 2 were observed at 3.50 and 3.20 ppm in the 1H NMR spectrum, while the outer NH proton was resonated at 8.95 ppm, suggesting the weak aromatic nature of the compound. Interestingly, by treating with SnCl2, 3 was transformed into a new aromatic macrocycle oxyindolophyrin 5 that showed the high affinity for fluoride anion (Scheme 3).24 On treating with CuII and AgI acetate, corresponding metal complexes 4a and 4b were obtained in quantitative yield.7a The 1H NMR analyses suggested that both are diamagnetic with +3 metal oxidation states. In the single crystal of 4a and 4b, the geometry around the metal center is nearly square-planar and 1D chain networks, in which VOL. 38, NO. 1, 2005 / ACCOUNTS OF CHEMICAL RESEARCH 11


FIGURE 1. One-dimensional networks of CuIII complexes of (a) cis-N2CP (4a) and (b) trans-N2CP (7). catalyzed condensation reaction of pentafluorophenylsubstituted N-confused dipyrromethane monocarbinol, followed by oxidation and base treatment (Scheme 4). The ethoxy group was derived from the alcoholic KOH.8 In contrast to the weak aromatic character of 2, the two inner NH and two β-CH protons were observed at -2.73, -3.21 ppm and -4.34, -4.36 ppm, respectively, indicating an 18π aromatic character of 6. Supporting this, the absorption spectrum exhibited a sharp Soret-like band at 449 nm.

Scheme 3

the peripheral N and NH are connected to each other through the hydrogen-bonding interactions, are formed. The distances between the two parallel planes are 6.89 and 6.39 Å, and the metal-metal distances are 8.395 and 8.329 Å for 4a and 4b, respectively (Figure 1a).7b The other doubly N-confused porphyrin, trans-N2CP (6), was obtained in 53% yield from the [2+2] acid-

Using Cu(OAc)2, the corresponding metal complex 7 was formed in 59% yield (Scheme 4).8 Absence of inner CH and NH signals suggests that the obtained complex contains a diamagnetic CuIII in the core. The X-ray structure reveals that the square-planar complex is rather planar with a small deviation (0.06 Å) from a mean plane consisting of 25 core atoms compared with the distorted free base (0.30 Å). The complex 7 forms intermolecular hydrogen-bonding rodlike chains as comparable with the zigzag chains of 4a. Reflecting the linear structure, the nearest CuIII-CuIII distance of 7, 10.83 Å, is longer than 4a (Figure 1b).

Scheme 4

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FIGURE 2. X-ray structures of (a) N-fused porphyrin (8), (b) N-fused pentaphyrin (9-Y), and (c) doubly N-fused pentaphyrin (12). Mesoaryl groups are omitted for clarity in the side views (bottom).

N-Fused Porphyrin and Doubly N-Fused Pentaphyrin Our earlier observation that the confused pyrrole ring in 1 is tilted largely from the porphyrin plane, e.g., 26.9° in 1a6a and 42.4° in the inner NO2-substituted NCP,25a drove us to a further study on core modification with the expectation of more flipping of the confused ring. Interestingly, during the halogenation, we obtained a new type of aromatic porphyrinoid with a fused pyrrole tripenta-

cyclic ring in the macrocyclic core, N-fused porphyrin (NFP, 8),25b which was derived by the inversion of the confused pyrrole. The X-ray structure of 8 revealed the existence of a fused tripentacyclic ring in the porphyrin skeleton, where all of the nitrogens are inside the core (Figure 2a). Moreover, on treatment with nucleophilic base, 8 was returned to alkoxy-substituted NCP by reinversion of the pyrrole ring (Scheme 5).25c An NCP dimer connecting each other at both the inner carbon atoms was synthesized from an NFP dimer by this ring-opening reaction.26 The above successful transformation encouraged us to apply similar strategies to higher homologues in the expanded porphyrins. As the first target, we chose a pentapyrrolic porphyrin, pentaphyrin, because the mesoaryl pentaphyrin is shown to exist in the form of N-fused pentaphyrin.27a The first N-fused pentaphyrin was obtained from the Rothemund-type condensation reactions in 15% yield, by using pentafluorobenzaldehyde and pyrrole, followed by oxidation with DDQ (Scheme 6).27a Two products (9-Y and 9-R) were formed, and the yield changed greatly according to the amount of oxidants used. From 1H NMR analyses, formation of a fused tripentacyclic ring was suggested by the absence of one of the β-CH protons. It was further confirmed by the X-ray structural analysis of 9-Y, where the inward- and outward-pointing nitrogen atoms were present in the fused tripentacyclic ring (Figure 2b), which is in contrast with the inwardpointing nitrogen atoms of 8. Therefore, the N-fused porphyrinoids can be classified as normal-type (9-Y) and confused-type (8) based on the linkage of the pyrrole rings. At this point, it was really a question whether the introduction of a confused pyrrole ring into such a macrocyclic system lead to a similar fused one or hitherto unknown N-confused pentaphyrin. To answer this question, we tried a [3+2] acid-catalyzed condensation reaction of N-confused tripyrrane and dipyrromethane dicarbinol, followed by oxidation, which afforded 10 in 40% yield.27b 1H NMR analysis suggested the presence of a fused tripentacyclic ring in the nonaromatic pentaphyrin 10.

Scheme 5

Scheme 6

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Scheme 7

(12).27b If we introduce one more confused pyrrole to such a macrocycle, which we call the doubly confusion approach, we do expect the hitherto unknown doubly N-confused pentaphyrin. Interestingly, the obtained pentaphyrin was not fused but oxo-substituted at the R position of the confused pyrrole rings. In this section, we described such pentaphyrins and their metal complexes. Stirring a mixture solution of N-confused tripyrrane and N-confused dipyrromethane dicarbinol through a [3+2] acid-catalyzed condensation, followed by p-chloranil oxidation, afforded two isomeric, partially oxidized products 13a and 13b in 23 and 12% yields, respectively, along with a trace amount of 14 (path a of Scheme 8).27c The observed 1H NMR spectra suggested that both isomers were symmetrical pentaphyrinogen intermediates, which were rather unstable and easily oxidized into aromatic 14.

Scheme 8

Further oxidation led to the quantitative formation of aromatic compound 11. However, 11 was unstable in CH2Cl2 and gradually isomerized into doubly N-fused pentaphyrins 12 (Scheme 7).27b The isomerization took place with the loss of aromatic stabilization of the pentaphyrin core and turned into nonaromatic. This was reflected from the 1H NMR and the electronic spectral analyses. The explicit structure of 12 was revealed by X-ray single-crystal analysis (Figure 2c). There are two tripentacyclic rings and a pyrrole ring in the macrocycle. Both the nitrogen atoms in one of the fused rings are inside the core (like 8), while in the other, the outward- and inward-pointing nitrogen atoms are found in the macrocyclic core (like 9-Y). The core plane is highly distorted from the least-squares plane, with a mean deviation of 0.413 Å. Each asymmetric unit of the crystal data contains one pentaphyrin with a cocrystallized acetonitrile solvent molecule. Similar to 8 and 9-Y, the inner proton shows an intramolecular hydrogen-bonding interaction with an adjacent pyrrolic nitrogen atom.

Doubly N-Confused Pentaphyrin and Hexaphyrin In the N-confused pentaphyrin chemistry, we already observed the N-confused, doubly N-fused pentaphyrin 14 ACCOUNTS OF CHEMICAL RESEARCH / VOL. 38, NO. 1, 2005

By changing the oxidizing agent from p-chloranil to DDQ, the doubly N-confused oxo-pentaphyrin (14) was obtained as a green solid in 32% yield (path b of Scheme 8).27c However, the compound was relatively unstable and gradually oxidized into doubly N-confused di-oxo-pentaphyrin (15) in CH2Cl2 at room temperature over 7 days in 45% yield (path c of Scheme 8). The presence of monoand di-oxo groups was confirmed by fast atom bombardment mass spectrometry (FABMS) and IR measurements. The 1H NMR spectra of unsymmetrical 14 and symmetrical 15 were proved as 22π aromatic. Supporting this, the electronic absorption spectra of 14 displayed a Soretlike band at 544 nm, while in 15, the band was observed split at 542 and 585 nm. The X-ray crystal analyses of 14 and 15 confirm that the confused pyrroles are inverted and one oxygen atom is attached at an inner R carbon in 14 (Figure 3a), while two oxygen atoms are present in 15 (Figure 3b). The mono-oxo-pentaphyrin (14) plane consisting of 30 core atoms is highly distorted from the least-squares plane with a mean deviation of 0.32(1) Å. There are three inner NH protons: two of them are forming intramolecular hydrogen bonding with the neighbors, while the remaining NH proton interacts with its countered moiety to form a dimer through the intermolecular hydrogen bondings. In contrast, the two CONH units in 15 interact intramolecularly with its neighbor. Because of such hydrogen bondings, the pentaphyrin core is less distorted [mean deviation ) 0.270(7) Å] as compared with 14. These are the first examples of stable non-fused pentaphyrins of meso-aryl type. Here, it is clear that the introduction of one more confused pyrrole brings the remarkable structural diversity.14,27 The metal coordination chemistry was examined on stable 15 by using RhI salt. By treatment of 15 with [Rh(CO)2Cl]2 in refluxing CH2Cl2, the RhI complex (16) was obtained in quantitative yield (path d of Scheme 8).27c An unsymmetrical structure of 16 was suggested by 1H NMR analysis, and the electronic absorption spectrum displayed the bathochromic shift of the Soret and Q bands (40 and 46 nm) as compared with 15. The X-ray analysis of 16 confirmed the presence of the Rh atom in the pentaphyrin core, which is located above the mean plane, and the


of 17a and 17b were largely improved (Scheme 9).28c,d The structure of 17b was confirmed by X-ray analysis, where two pyrrole rings were inverted and distorted above and below the macrocyclic plane with a mean deviation of 0.536(8) Å. The confused derivatives of hexaphyrin were recently synthesized.29 When N-confused tripyrrane was treated with pentafluorobenzaldehyde in the presence of acid catalyst, followed by oxidation with p-chloranil, doubly N-confused hexaphyrins (18a and 18b) were obtained as a brown solid in 7 and 3% yields, respectively (path a of Scheme 10). Both nonaromatic 28π and aromatic 26π compounds were identified by 1H NMR and electronic spectral analyses. However, the aromatic 18b was unstable and gradually oxidized into the di-oxo derivative (19b) in CH2Cl2 solution at room temperature.

FIGURE 3. X-ray structures of (a) doubly N-confused mono-oxopentaphyrin (14), (b) doubly N-confused di-oxo-pentaphyrin (15), and (c) the RhI complex (16). Meso-aryl groups are omitted for clarity in the side views (bottom). geometry around the metal center is close to squareplanar [N-Rh-N, 80.2(2)°] (Figure 3c). To introduce one more metal ion or heavier metal ions in the core, the higher homologue, hexaphyrin, is attractive, because of its bigger core size. The first meso-aryl hexaphyrin of normal type was reported by Dolphin et al.28a But the poor stability of the compound hampered further spectroscopic characterization. The stable hexaphyrins (17a and 17b) were synthesized by Cavaleiro and co-workers through the modified Rothemund reaction in 1% yield.28b Using the oligopyrrole precursors, the yields

On varying the oxidizing agent from p-chloranil to DDQ in the condensation reaction, two different products (19a and 19b) were obtained as greenish solid in 7 and 10% yields (path b of Scheme 10).29 FABMS and IR analyses indicated the presence of two oxo groups. The outer β-CH and NH signals of confused pyrrole and the NH signals of normal pyrrole in 19b were resonated at 10.74, -0.73, and -0.24 ppm, suggesting the symmetrical structures with aromatic nature. Supporting this, the absorption spectrum displayed an intense Soret-like band at 566 nm. The X-ray analysis of 19b revealed that the confused pyrrole rings are inverted and the two oxo groups are attached at the inner R-carbon atoms affording the CONH groups (Figure 4a). Both the carbonyl groups are forming the intramolecular hydrogen bonding with adjacent pyrroles, keeping the molecule planar. The mean deviation is only 0.054 Å to a mean plane consisting of 36 core atoms. The doubly N-confused porphyrin 18 (and 19) contains the two N3C (N3O) compartments that facilitate the formation of bismetal complexes when it binds metals inside the core. Actually, the stable bis-CuII complex 20 was obtained by treating 19b with CuII acetate in CH2Cl2 solution in quantitative yield (path c of Scheme 10).29 The complex was paramagnetic, and the magnetic susceptibility measurements proved that the Cu ions were present in the paramagnetic d8 form (µeff ) 1.73 µB, at 77 K). The X-ray analysis of 20 confirmed the bismetal coordination and planarity of the molecule (Figure 4b). In contrast, the bis-Ni complex 21a was synthesized by a refluxing solution

Scheme 9



Scheme 10

Scheme 11

because of the solvent coordination (21b). This was further confirmed by X-ray structural analysis (Figure 4c).29

Confused Pyrrole in the Coordination Chemistry

FIGURE 4. X-ray structures of (a) doubly N-confused di-oxohexaphyrin (19b), (b) bis-CuII complex (20), and (c) bis-NiII complex (21b). Meso-aryl groups are omitted for clarity in the side view (bottom). of 19b in toluene by using NiII salt in 95% yield (path d of Scheme 10). The 1H NMR spectrum suggested that the compound was diamagnetic and the absorption spectrum displayed the broad bands, indicating that the complex was distorted largely from planarity. Interestingly, the observed broad bands were turned into sharper bands to afford a more porphyrin-like spectrum in CH3CN, suggesting the large structural change from distorted to planar 16 ACCOUNTS OF CHEMICAL RESEARCH / VOL. 38, NO. 1, 2005

The first example of a metal-carbon bond in NCP (1) was shown by Latos-Graz˘ yn ´ ski et al. with a divalent NiII square-planar complex.6b The trianionic property of 1 was demonstrated with AgIII complex by us.30a Since then, a series of dianionic and trianionic metal complexes have been synthesized.30b The first outer nitrogen coordination was proved by the RhI complex of 1. When 1 was treated with Rh[(CO)2Cl]2, bis-RhI complex 22 was obtained in 53% yield (Scheme 11).31a In this section, we focused on the role of confused pyrrole in the outer coordination and anion-binding chemistry. Using Pd(OAc)2, in the presence of 1 in toluene, along with the inner-coordinated monomer PdII complex, two more new dimer complexes (23a and 23b) were also obtained in 27 and 36% yield, respectively (path a of Scheme 12).31b In both complexes, two NCP units coordinated to two Pd atoms forming cyclic dimers. The geometry around one of the Pd metal centers is squareplanar, while the other is distorted to tetrahedral (Figure 5a). The difference between the dimer complexes is the orientation of two NCP units, which is revealed by X-ray analyses.


aqueous Et4NOH solution led to the formation of a brownish dinuclear dimer (26a).32a Like 26a, CdII (26b) and HgII (26c) complexes were synthesized under similar conditions (path b of Scheme 13). In 26a, two ZnII metals were coordinated with the inner N3C atoms and an outer nitrogen of the other NCP paired.32b Interestingly, the structures are similar to the reported MnII 32c and FeII 32d dimer complexes except for the relative orientation of the confused pyrrole rings. In the crystals, two confused pyrrole rings in 26a (and 26b) are tilted by 47.5(3) and 49.9(3)° [41.3(5) and 42.9 (5)°] from the corresponding N3 mean plane comprising three inner nitrogen atoms. The distances between the two metal centers are 5.217 (2) Å for 26a and 5.230 (2) Å for 26b (parts a and b of Figure 6).

Confused Pyrrole in Anion-Binding Chemistry FIGURE 5. X-ray structures of NCP dimer complexes of (a) bis-PdII (23b), (b) cis-PtII (24a), and (c) trans-PtII (24b). Meso-aryl groups are omitted for clarity. In continuation of the PdII complexes, we applied the similar strategies to the PtII complexes. When N-confused tetra(p-tert-butylphenyl)porphyrin (1-tBu) was treated with 0.5 equiv of PtCl2 in refluxing toluene for 1 h, greenish dimer complexes 24a and 24b were obtained in 69 and 5% yield, respectively, along with a trace amount of the monomer complex (path b of Scheme 12).31c The X-ray structures revealed that the Pt atom is connected in a square-planar fashion with two outer nitrogen atoms of NCP molecules, an ortho-carbon of the meso-aryl group and a chloride atom. Two confused pyrrole rings are arranged in cis (24a) and trans (24b) configurations and tilted 20.94° for 24a and 28.72° for 24b to the mean planes consisting of 24 core atoms (parts b and c of Figure 5). When 1 was treated with 2 equiv of Zn(OAc)2‚2H2O in CH2Cl2 at room temperature, green color crystals of tetraZnII-coordinated dimer (25) were obtained quantitatively (path a of Scheme 13). Subsequent treatment with 1%

Aside from the outer metal coordination chemistry, the peripheral nitrogen is also essential for binding anions. The first such example was demonstrated with a SbV complex. When 1 was treated with 10 equiv of SbBr3, anion-bound crystals (27a) were obtained in 42% yield. When a slight excess of pyridine was added to 27a, neutral complex 27b was obtained in quantitative yield. Similarly, by adding HBr to 27b, 27a was retained (Scheme 14a).33a The X-ray crystal analysis of 27a confirmed that a bromide ion is bound at the outer nitrogen with a distance of 3.771(9) Å, which is within a range of the hydrogen-bonding interaction (Figure 7). These results encouraged us to investigate the anion binding at the peripheral nitrogen of other NCP metal complexes. Recently, we synthesized the CuII (28a) and CuIII (28b) NCP complexes (Scheme 14b).33b Interestingly, electrochemical interconversion between these two complexes was controlled by the addition of anions. The large negative potential shift observed for Cl- [0.03 V as compared to ClO4- (0.14 V) and PF6- (0.15 V)] might be attributable to a hydrogen-bonding interaction with the outer NH of the confused pyrrole ring. Transmission of the partial negative charge from the peripheral hydrogen-

Scheme 12



Scheme 13

FIGURE 6. X-ray structures of NCP dimer complexes of (a) bis-ZnII (26a) and (b) bis-CdII (26b). Scheme 14

FIGURE 7. X-ray structure of HBr salt of antimony(V) NCP complex (27a). A broken line indicates the hydrogen-bonding interaction.

Efficient Sensitizers for Singlet-Oxygen Generation Nowadays, there is a great interest in finding new molecules that can be used as sensitizers in PDT.16 Porphyrins and their analogues are being extensively studied; however, the majority of the porphyrins have low absorptivities in the phototherapeutic window (about 600-1000 nm), where light can be more effectively used, because it can penetrate deeper in the tissues.16 The cis-N2CP (2) and its Cu (4a) and Ag (4b) complexes exhibit relatively intense absorption bands around 660 nm, and a remarkable discovery is that the complex 4b is a very efficient singletoxygen sensitizer in CH3CN solution.34

bonding anion, N-H‚‚‚X-, to the Cu cation through the inner carbon of the confused pyrrole ring might be occurring. It is noteworthy that the estimated Cl--binding affinity, 4.9 × 104 M-1 is unusually high for a single hydrogen-bonding system (Br-, ClO4-, and PF6- in CH2Cl2 were determined as 6.9 × 103, 360, and 50 M-1, respectively, from the absorption spectral changes); thus, the contribution of a zwitterionic resonance form of 28a and an additional interaction between Cl- and the electrondeficient C6F5 group nearest to the outer NH were suggested in this anion-binding system.33b,c 18 ACCOUNTS OF CHEMICAL RESEARCH / VOL. 38, NO. 1, 2005

The photophysical properties of 2, 4a, and 4b were examined by means of electronic and time-resolved spectroscopy, inferring that the fluorescence of the metal complexes is being suppressed by the enhancement of the intersystem crossing, induced by the heavy-atom effects. The singlet-oxygen quantum yield of 4b (φ∆ ) 0.92) was higher than 2, 4a, TPP, and monoacidic benzoporphyrins (0.2, 0.66, 0.73, and 0.76). Because of the relatively redshifted absorption bands (613 and 665 nm), molar absorptivities (log ) 4.2, 3.9), and good thermal and photochemical stability, 4b becomes the first unusual valence metal complex with suitable features as a PDT photosensitizer. Aside from cis-N2CP, singlet-oxygen gen-


eration upon irradiation above 700 nm was demonstrated with N,N′-dimethylated NCP by Dolphin and co-worker.35

Summary and Outlook The serendipitous discovery of N-confused porphyrin disclosed some new aspects apart from confusion. By the “single confusion approach”, we found inversion followed by fusion. The unique confused pyrrole in NCP exploited a series of outer nitrogen coordination, particularly bound with the second and third row transition metals as well as various anions. The metal valencies are controlled by outer stimuli, which may offer a variety of applications, including molecular switches and anion sensors. The long wavelength absorption of fused derivatives might be potentially useful as NIR materials. By the "double confusion approach", we synthesized the second generation of NCPs called cis- and trans-N2CP. These ligands can stabilize the higher oxidation states of metals by forming two metal-carbon bonds, which would be useful in employing as organometallic reagents. The presence of acid (protonated nitrogen) and base (deprotonated nitrogen) moieties at the periphery would make these “imidazole porphyrinoids” attractive for the use as acid catalysts and anion/cation sensors. Formation of supramolecular assemblies in the solid state is of much interest for preparing functional materials. The efficient singlet-oxygen sensitization of such macrocycles is appealing in using as PDT agents. In the expanded porphyrin area, besides the structural diversity, we found one more important confused ring of oxo type. The availability of bigger cavities would provide attractive bismetal complexes for the study of optoelectronic and optomagnetic materials. Introduction of further confusion in oligopyrrolic macrocycles, therefore, can be interpreted, in some sense, to activate macrocycles for further mutation. The future of the "confusion approach" toward novel porphyrinoids thus looks especially bright particularly in the areas of 2D and 3D supramolecular architectures. The authors are grateful to the postdoctoral fellows, graduate students, and collaborators whose names are cited in the references for their invaluable contributions. A. S. thanks JST, JSPS, and COE program for the postdoctoral research fellowship. This work was partially supported by Grants-in-Aid from the PRESTO (Synthesis and Control), Japan Science and Technology Agent, and the 21st Century COE Program, Functional Innovation of Molecular Informatics Kyushu University on Molecular Informatics, from the Ministry of Education, Culture, Science, and Technology, Japan.

Note Added after ASAP Publication. This article was initially published ASAP on December 3, 2004, with errors in Chart 2 and Schemes 3, 10, 12, and 13. The corrected version was published ASAP on December 8, 2004.

References (1) The Porphyrin Handbook; Kadish, K. M.; Smith, K. M.; Guilard, R. Eds.; Academic Press: San Diego, 2000; Vol. VI, Chapter 4046.

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Confusion Approach to Porphyrinoid Chemistry - ACS Publications

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